1. Field of the Invention
This invention relates to a seal system designed for particular use in the completion and production operations of oil and gas wells wherein the seal comprises a plurality of plastically deformable members.
2. Description of the Prior Art
Petroleum reserves found in relatively deep wells and in relatively hostile environments have resulted in attendant problems due to sour formation fluid, high pressure and increasingly higher temperatures. Under these conditions, a reliable downhole seal system must function in hot, high pressure, sour gas environments, including the presence of corrosive materials such as hydrogen sulfide. These considerations, especially the chemical environment in which the seal must operate, may limit the choice of packing materials to a plastically deformable or nonelastomeric element having relatively low resiliency and high permanent set, particularly when sealing against extremely high pressures. Polytetrafluoroethylene, commonly referred to under the DuPont trademark as "Teflon", is one material having the necessary chemical resistance, but also has relatively low resiliency and high permanent set. Teflon will, therefore, not have the flexible characteristics which make elastomers, including perfluoroelastomers, a highly attractive sealing material for use in certain hostile environments.
Polytetrafluoroethylene members have been used as sealing or packing elements in both subterranean oil well applications and for providing sealing engagement to cylindrical members in other applications. For example, Teflon packing elements have been utilized for external packer-to-casing seal systems where the packing element is subjected to compressive preloading which energizes the element and forces it radially outward to establish sealing engagement with the casing. Unlike these Teflon external packer-to-casing seal elements, the present invention employs self-energizing chevon-shaped Telfon sealing elements. The distinction between a preloaded sealing element and a self energizing chevron ring sealing element is demonstrated by one patent intended for use in a non oil well environment. U.S. Pat. No. 3,351,350 depicts a high pressure rod seal employing a preloaded polytetrafluoroethylene ring used independently of a separate chevron ring made from neoprene impregnated duck fabric.
The prior art use of polytetrafluoroethylene in a chevron-shaped configuration illustrates the overriding need of providing means to prevent axial extrusion of the chevron-shaped Teflon members and to enhance the elastic memory of Teflon elements. The use of a plurality of chevron-shaped glass-filled Teflon members in combination with a two-piece titanium metal-to-metal anti-extrusion ring was reported in Report No. SPE 6762 of the 52nd Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers on Oct. 9th through the 12th, 1977. Satisfactory performance of the chevron-shaped Teflon members was achieved only in conjunction with the two-piece metal-to-metal anti-extrusion ring. The alternative problem of imparting elastic memory to the Teflon member is disclosed in U.S. Pat. No. 3,467,394. That patent discloses a packing assembly employing Vee-shaped polytetrafluoroethylene sealing elements interspersed with relatively rigid packing rings. The packing rings are made from flexible materials, including various flexible types of elastomers and plastics which are used to impart their memory to the Teflon sealing rings. A packing ring comprising alternating rings of thermosetting material and thermoplastic material, such as Teflon, is disclosed in U.S. Pat. No. 3,627,337. The more or less rigid non-flowing thermosetting materials were found to restrain the thermoplastic components against excessive flow.
Despite its physical limitations, Teflon would be a desirable sealing element for use in an oil well packing element system if only because of its resistance to chemicals encountered during production of fluids in an oil or gas well. The present invention provides such a means for utilizing chevron-shaped self-energizing Teflon sealing elements to provide an acceptable seal at high temperature and pressure. Virgin polytetrafluoroethylene seal elements are used in conjunction with twenty-five percent glass-filled polytetrafluoroethylene back up elements and polyphenylene sulfide back up elements, both of which have a greater resistance to extrusion than virgin polytetrafluoroethylene. Polyphenylene sulfide is commonly referred to as "Ryton", a trademark of Phillips Petroleum Corporation. Glass-filled Teflon back up members and Ryton back up members have been used in conjunction with perfluoroelastomer elements, commonly referred to under the DuPont trademark "Kalrez", to provide an unusually effective packing element for high temperature and high pressure environments. The chemical resistance of Teflon is, however, generally better than that of Kalrez, especially to certain amine corrosion inhibitors into producing wells. U.S. Pat. No. 4,234,197 discloses the use of "Kalrez" elements with Ryton and glass-filled Teflon members.
Teflon has also been used in conjunction with Ryton to form a single sealing element. The use of polyphenylene sulfide seals impregnated with Teflon elements is disclosed in U.S. Pat. No. 4,050,701.
The seal system provided by the present invention employs only plastically deformable members, such as Teflon, to establish a seal between concentric conduits in a subterreanean oil well. Elastomeric elements exhibit substantially complete memory while plastic elements do not. By replacing the elastomeric sealing elements with a nonelastomer, the temperature range over which the seal system performs adequately is somewhat less than can be achieved with elastomeric systems. The use of a nonelastomer as the primary sealing element limits the lower temperature at which an acceptable seal can be maintained. After the thermoplastic Teflon material is initially heated to some higher temperature, the loss of memory inherent in the nonelastomeric material prevents acceptable sealing performance at ambient temperatures. Normally, however, the self energizing Teflon elements would be initially heated to a temperature on the order of 400.degree. F. upon insertion into a hostile environment well. Temperatures would not be expected to return to ambient conditions and the temperature cycling encountered in the majority of treating jobs would permit the use of this nonelastomeric sealing system even though it does not completely return to its initial state.